There are more than 100 known herpesviruses in the family of Herpesviridae. Of these, eight are known to infect humans. The eight human herpesviruses are herpes simplex virus 1 (HSV-1); herpes simplex virus 2 (HSV-2); varicella -zoster virus (VZV); Epstein-Barr virus (EBV); cytomegalovirus (CMV); herpesvirus 6 (HHV-6); herpesvirus 7 (HHV-7); and herpesvirus 8 (HHV-8), also known as Kaposi's sarcoma associated herpesvirus (KSHV).
Based on the length of viral replication cycle and host tissue range, the herpesviruses are classified into three sub-families, alpha-, beta-, and gamma-herpesviruses. Following primary infection, all herpesviruses establish latent persistent infections within tissues characteristic for each virus. For example, the alpha-herpesviruses HSV1, HSV2 and VZV are neurotropic, while EBV, CMV, HHV6, HHV7 and HHV8 are lymphotropic.
Human herpesvirus infections are very common and widely distributed. Serologic surveys indicate that >95% of adults worldwide have been infected by VZV, EBV, and HHV-6. Despite a vigorous anti-viral immune response, herpesviruses persist in the host following primary infection. This asymptomatic latent period may be interrupted by periods of viral reactivation during which virus replicates and clinical symptoms may occur. Examples include recurrent cold sores (HSV-1), herpes zoster (shingles) in older adults arising from VZV acquired during childhood (chicken pox), CMV pneumonitis in immunocompromised organ transplant patients, and recurrent mononucleosis in patients with chronic (EBV) mononucleosis syndrome.
In many cases, the diagnosis of herpesvirus infection cannot be accurately made by clinical findings alone. Symptoms are often nonspecific, e.g., fever, malaise, lymphadenopathy, and rash. Patients can sometimes be infected with more than one herpesvirus (e.g., frequent association of HHV-8 and EBV in primary effusion lymphoma, HSV-1 and HSV-2 in orogenital ulcers) . Whereas infections with the CMV are usually amenable to acyclovir or gancyclovir anti-viral treatment, no suitable effective drug treatment is available for EBV, HHV-6, HHV-7, and HHV-8. Thus, identification of specific human herpesvirus infection is necessary before proper therapy can be selected.
The pathogenesis and clinical importance of the recently identified lymphotropic viruses HHV-6, HHV-7 and HHV-8 are not well understood. A better clinical understanding of these viruses requires the availability of appropriate diagnostic approaches for their detection and identification. All these factors, along with the worldwide impact of human herpesvirus infection, demonstrate the need for a reliable multiplex clinical assay for the detection and identification of human herpesviruses. Although a clinical assay need not differentiate EBV-1 from EBV-2 or HHV-6A from HHV-6B, it should be able to detect variants and to distinguish each type of herpesvirus from the other herpesviruses.
Current laboratory techniques for detection of herpesvirus infection include virus culture, viral serology, and viral DNA detection by molecular diagnostic methods such as PCR. Given the considerable limitations of culture and serology for herpesvirus detection, PCR detection methods have been developed. Molecular diagnostic methods such as PCR offer the distinct advantages of rapid turn-around time, high sensitivity, and high specificity for the detection of herpesvirus infections.
Several general approaches have been used to identify one or more types of human herpesvirus. Rozenberg and Lebon ((1991) J. Clin. Microbiol. 29(11):2412-7) describe a single-step PCR assay using a consensus primer pair for HSV-1, HSV-2, EBV, and CMV, followed by typing of the amplicons by restriction fragment length polymorphism analysis (RFLP). However, the complexity of RFLP restricts its use to sophisticated laboratory environments.
Tenorio, et al. ((1993) J. Virol. Meth. 44:261-9) disclose a nested amplification method for detecting herpesvirus. Non-degenerate oligonucleotides were used to generate a first set of amplicons which served as a substrate for a second, multiplex reaction for which primers were designed to produce different-size fragments for each related virus. However, because this method requires two separate, sequential PCR reactions, there is a significant risk of contamination of PCR reactions which leads to false positive reactions. Further, Elfath, et al. ((2000) Clin. Microbiol. Rev. 13:559-570) teaches that whenever possible, multiplex PCRs should avoid the use of nested primers requiring a second round of amplification and precautions should be taken to avoid false-negative results due to reaction failure.
Aono, et al. ((1994) Acta Otolaryngol. Suppl. 514:132-4) described a multiplex PCR using consensus primers for the detection of the three herpesviruses (HSV-1, HSV-2 and VZV) by virus-specific probe hybridization. Unlike restriction fragment length polymorphism, virus-specific probe hybridization is more compatible with clinical laboratory requirements of short cycle time and simplicity.
McElhinney, et al. ((1995) J. Virol. Methods 53:223-233) teach a three-target PCR method for the investigation of latent and active CMV and HHV-6 infection, and the use of human beta-globin DNA in the same reaction to avoid false-negative DNA amplifications for CMV.
van Devanter, et al. ((1996) J. Clin. Microbiol. 34(7):1666-71) developed a set of degenerate consensus primers for PCR amplification of conserved regions of the DNA polymerase gene. The resulting nested consensus primer PCR method allowed for amplification and identification of most (14 of 15) of the animal herpesviruses and 6 of 8 human herpesviruses (HHV-1, HHV-2, VZV, EBV, CMV, HHV-6B). The method did not amplify human DNA polymerase, or yeast/mold DNA polymerase that are common contaminants of human samples. However, the methodology exhibited a wide variation in sensitivity across the human herpesviruses tested. The LOD (limit of detection) varied between 1 copy per 100 ng DNA for HSV-1 and HSV-2, and 100 copies for EBV and VZV. No data was presented on amplification of HHV-7 or HHV-8. Each virus was identified by direct DNA sequencing of the amplified products obtained from an ethidium bromide-stained agarose gel. This method of DNA sequence typing is a highly complex, laborious method not appropriate for use in a clinical diagnostic laboratory. Moreover, this reference does not teach that the method can identify more than one herpesvirus in a single sample.
Colimon et al. ((1996) J. Virol. Methods 58:7-19) developed the use of “stair primers” to allow PCR amplification of viral genomes with frequent point mutations, such as HIV and hepatitis C virus. Minjolle, et al. ((1999) J. Clin. Microbiol. 37:950-3) adopted the use of these “stair primers” for herpesvirus assay, utilizing mixtures of consensus stair primers to amplify DNA polymerase for the detection of six of the eight human herpesviruses (HHV-1, HHV-2, VZV, EBV, CMV and HHV-6). However, amplicons were detected by virus-specific probe hybridization with chromogenic detection.
Ehlers, et al. ((1999) Virus Genes 18:211-220) developed an enhanced version of the van Devanter method. This reference noted that the van Devanter method exhibited a wide variation in binding of the degenerate primers to different herpesviruses and therefore deoxyinosine (dI) was substituted at degenerate positions within the van Devanter primers. DNA polymerase of some herpesviruses were not amplified at all by the dI-substituted primers (e.g., CMV). Using a mixture of dI-substituted and unsubstituted primers, it was found that the mixed primer set improved overall performance for herpesviruses from a range of species. Ehlers demonstrated that six of the eight human herpesviruses (HSV-1, HSV-2, VZV, EBV, CMV and HHV-8) could be amplified by this method, while reducing the virus-related variability in the limit of detection. However, the important issues of assay complexity and turn-around time were not addressed since Ehlers, like van Devanter, intended to utilize the assay primarily to support research rather than clinical analysis.
Pozo and Tenorio ((1999) J. Virol. Methods 79:9-19) developed a two-step consensus primer PCR assay for the six lymphotropic human herpesviruses. Six pairs of primers were used in a first PCR step to produce a virus-specific 194-bp amplicon of the DNA polymerase gene. Then six pairs of primers were used in a second PCR step in which the reverse primer targets a highly conserved region of each amplicon, and the forward primer governs a difference in amplicon size (e.g., 54-122 bp). Subsequent gel electrophoresis with ethidium bromide-staining allowed typing of each band by its migration rate on the gel. A limit of detection of 10-100 copies for the six lymphotropic herpesviruses was reported.
Johnson et al. ((2000) J. Clin. Microbiol. 38(9) :3274-9) developed a two-step PCR-based assay for detection and species identification of human herpesviruses. Two consensus primer pairs were used, one for the three α-herpesviruses, the other pair for the five β- and γ-herpesviruses. The primer pairs bracketing a highly conserved region of the DNA polymerase gene allowed amplification of all eight major human herpesviruses at a limit of detection of 10-100 copies, with the exception of CMV that had a limit of detection of 400 copies. Agarose gel electrophoresis was used for visual identification of fluorescent ethidium bromide-stained bands to identify sample amplicons positive for human herpesvirus. Positive amplicon reaction mixtures were then subjected to two separate restriction endonuclease digestions (BamHI and BstUI) . The restriction digests were then subjected to agarose gel electrophoresis and the human herpesvirus species was identified based on the restriction fragment patterns (RFLP) on the two gels. However, the use of dual RFLP is complex and time-consuming for the clinical laboratory.
Yamamoto and Nakamura ((2000) J. NeuroVirol. 6:410-417) teach two sets of degenerate primers (P1 and P2, P3 and P4) specific for DNA polymerase from different herpesviruses. The resulting amiplicons (516, 514, 588, 510 and 522 bp corresponding to HSV-1/-2, VZV, CMV, HHV-6 and EBV, respectively) were separated on an ethidium bromide-stained gel and subsequently probed to confirm specificity of the PCR reaction.
Markoulatos et al. ((2000) J. Clin. Lab. Anal. 14:214-9) disclose a multiplex PCR assay for the detection of HSV-1, HSV-2 and VZV; however, this assay does not detect CMV, EBV or HHV-6. Subsequently, this assay was modified to also amplify CMV and EBV, however, the primer set did not amplify HHV-6 (Markoulatos, et al. (2003) J. Clin. Lab. Anal. 17:108-112). Additional multiplex assays have been disclosed; however, these references fail to teach detection of more than three types of herpesviruses (Cassinotti, et al. (1996) J. Med. Virol. 50(1):75-81; Read and Kurtz (1999) J. Clin. Microbiol. 37(5) :1352-5; O'Neill, et al. (2003) J. Med. Virol. 71(4) :557-60; Druce, et al. (2002) J. Clin. Microbiol. 40(5) :1728-32).
Robert, et al. ((2002) J. Med. Virol. 66:506-511) used a commercially available kit based on stair primers in a two stage multiplex PCR assay of six herpesviruses (HHV-1, HHV-2, VZV, EBV, CMV, and HHV-6) in tear fluid. Samples were amplified using the Argene HERPES CONSENSUS GENERIC™ Kit. Amplicons positive for herpesvirus were typed with the Argene HERPES IDENTIFICATION HYBRIDOWELL™ Kit by virus-specific probe hybridization with a chromogenic substrate for detection.
U.S. patent application Ser. No. 10/641,665 discloses assays for the detection and typing of ten human herpesviruses. The assays involve nested multiplex PCR using consensus primers to amplify conserved regions of the herpesvirus DNA. A dot blot/chemiluminescence assay and real-time PCR assay are disclosed as is a heteroduplex mobility assay.
Thus, the prior art is deficient in a one-step PCR-based assay capable of detecting and typing the major human herpesviruses in a clinical setting. The present invention fulfills this long-felt need.